Microcapsules in which the core is a single liquid droplet and the shell is a fusible material are known. (See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, 3d ed., Vol. 15, pages 473-474.) The microcapsules described there are produced by a process developed by the Southwest Research Institute, in which the core phase of material and the shell phase are separately liquefied and sprayed through concentric nozzles, forming molten droplets in which the shell phase encapsulates the core phase. The droplets are cooled, as with a countercurrent of air, to form solid particles.
Microcapsules made by the Southwest process have a solid shell and a single liquid core (like eggs), so they are very susceptible to rupturing. When many microcapsules are collected to form a bulk powder, the rupture of a small proportion of the microcapsules can reduce a free-flowing powder of the microcapsules to a sticky mass which no longer flows freely. Such a material thus cannot be handled in bulk with any facility. An additional disadvantage of the Southwest process is the need for very precise control of orifice sizes, pump pressures, and temperatures, so production costs are high and production rates are low.
Microcapsules in which the phases are reversed, the inner phase being an oily material and the outer phase being an aqueous suspension or solution which is dried by spray drying to form a solid coating, are also known. (See U.S. Pat. Nos. 3,819,838, issued to Smith, et al. on Jun. 25, 1974; 2,785,983, issued to McMath on Mar. 19, 1987; 2,857,281, issued to Schultz, et al. on Oct. 21, 1958; and 3,764,346, issued to Noznick, et al. on Oct. 9, 1973.)
Other prior art describes solid particles which are coated with a fusible material. (See, for example, U.S. Pat. Nos. 4,173,992, issued to Pollard on Nov. 6, 1979; 3,856,699, issued to Miyano, et al. on Dec. 24, 1974; 3,819,838 (cited before--shows overcoating a microparticle having a solid wall with a fusible material); and 4,675,236, issued to Ohkawara, et al. on Jun. 23, 1987). In most of the prior art, because the aqueous material is on the outside, and because water is readily removed from the aqueous material by spray drying or other means, a spray cooling process is not usually used to form the microparticles.
It is also well known that microcapsules can be made by preparing an emulsion and spraying the emulsion to divide it into particles. However, this process has conventionally been used with oil-in-water emulsions. Being the continuous phase, the water solution forms the outside of the capsule.
Finally, the Kirk-Othmer citation previously set forth, page 476, describes the emulsification of water into melted fat or wax, which emulsion is then subdivided into a discontinuous phase of a second emulsion, the continuous phase of which is cooled below the fusion temperature of the melted fat. Particles of the first emulsion are then separated from the water phase of the second emulsion. The result is a water phase encapsulated in congealed fat or wax. The reference does not state whether the water phase within each shell is subdivided into a multiplicity of cores.